Allow building the hit tester independently.

webrender/src/render_backend.rs

@@ -293,6 +293,24 @@ impl Document {
         }
     }
 
+    fn rebuild_hit_tester(&mut self) {
+        if let Some(ref mut frame_builder) = self.frame_builder {
+            let accumulated_scale_factor = self.view.accumulated_scale_factor();
+            let pan = self.view.pan.to_f32() / accumulated_scale_factor;
+
+            // TODO: do we need to do something with the transform palette?
+            let _palette = self.clip_scroll_tree.update_tree(
+                pan,
+                &self.dynamic_properties,
+            );
+
+            self.hit_tester = Some(frame_builder.create_hit_tester(
+                &self.clip_scroll_tree,
+                &self.clip_data_store,
+            ));
+        }
+    }
+
     pub fn updated_pipeline_info(&mut self) -> PipelineInfo {
         let removed_pipelines = replace(&mut self.removed_pipelines, Vec::new());
         PipelineInfo {